Experiments of microcrystalline cellulose conversion in subcritical and supercritical water were conducted at temperatures between 290 and 400degreesC, a pressure of 25 MPa, and residence times of 0.02-13.1 s using a continuous-flow-type microreactor. First, the reaction mechanism of microcrystalline cellulose in subcritical and supercritical water was proposed on the basis of detailed product analyses. Next, the kinetic description of this reaction in subcritical and supercritical water using a grain model was carried out to verify the proposed reaction mechanism and consequently found that the reaction-rate models were able to express the reaction of microcrystalline cellulose at identical conditions. (C) 2004 American Institute of Chemical Engineers